1. Field of the invention
The present invention relates to an electrostatographic toner employable for producing a visible image from a latent image in a recording method utilizing electrostatography.
2. Description of prior art
As processes for fixing a toner image in a recording method such as electrostatography, there have been known three fixing processes, that is, a heat fixing process, a solvent fixing process and a pressure fixing process. Recently, the heat fixing process and the pressure fixing process, both using no solvent, are widely used from the viewpoint of prevention of environmental pollution.
In the heat fixing process, a toner comprising a colorant and a binder has been conventionally employed. The same kind of toner is also employed in the pressure fixing process, and utilization of an encapsulated toner (i.e., toner in the form of micro-capsule) is recently proposed in the pressure fixing process.
The encapsulated toner is a toner in the form of micro-capsule prepared by enclosing a core material comprising a colorant such as carbon black and a binder such as a polymer and an oily solvent with a resin shell which is rupturable by application of a pressure, while the conventional toner is a toner comprising a binder and a colorant such as carbon black dispersed therein.
The conventional encapsulated toner and the conventional toner are not necessarily satisfactory in various properties essentially required for toners.
As a developing agent employable for electrostatography, there has been conventionally employed a toner having insulation property on its surface, but recently a toner provided with appropriate positive or negative friction-electrostatic chargeability on the surface has been developed.
The electrostatography comprises the steps of electrostatically charging a toner by rubbing the toner with carrier particles such as iron particles or fur, and causing a latent image to attract the electrostatically charged toner. Accordingly, the toner employable in the electrostatography is required to have positive or negative friction-electrostatic chargeability corresponding to the negative or positive electrostatic charge of the latent image depending on the nature of an apparatus employed.
For providing friction-electrostatic chargeability to the conventional toner, there is generally utilized a method of adding an electrostatic charge modifier to a binder resin in the preparation of a toner to disperse the electrostatic charge modifier inside the toner. As the electrostatic charge modifier, nigrosine is generally used to give positive friction-electrostatic chargeability to the toner and a metal complex dye is generally used to give negative friction-electrostatic chargeability to the toner. These modifiers practically serve as electrostatic charge modifiers only when they exist on the surface of the toner.
In the case of an encapsulated toner, a method of depositing a powdery electrostatic charge modifier such as silica powder on the surface of the encapsulated toner is utilized for the same purpose, because it is difficult to effectively fix the above-mentioned electrostatic charge modifier such as nigrosine or a metal complex dye on the surface of the encapsulated toner.
However, the present inventor has found that in any of the toner containing an electrostatic charge modifier such as nigrosine and the encapsulated toner deposited with an electrostatic charge modifier such as silica powder on its surface, the electrostatic charge modifier easily separates or drops off from the toner when the toner is stored or employed for a long period of time.
In the case that a relatively large amount of the electrostatic charge modifier separates or drops off from the toner, the toner tends to have insufficient developing property to cause unfavorable effect on the characteristics of the resulting visible image such as lowering of sharpness or decrease of optical density.
With respect to the friction-electrostatic chargeability of a toner, Japanese Patent Provisional Publication No. 59(1984)-151159 discloses an invention relating to a toner for developing an electrostatic image which is characterized in that to the surface of the toner are chemically attached charge-control groups.
In more detail, the toner disclosed in the above publication is a toner in which compounds having a charge-control group is chemically bonded to functional groups existing on the surface of the toner particle. The compound, unlike the electrostatic charge modifier deposited on the surface of a conventional toner, is chemically bonded to the surface of the toner so as not to easily separate or drop off from the toner, whereby the resulting toner can be provided with relatively stable friction-electrostatic chargeability. In this method, however, the toner can be hardly improved in other physical properties required for toners, though the toner is improved in friction-electrostatic chargeability owing to the charge-control group chemically bonded to the toner surface. Hence, in order to improve other various properties, other components than the above-mentioned compound should be still employed according to the conventional manner.
A toner is required to be excellent in various properties such as powder flowability and preservation stability as well as the friction-electrostatic chargeability. If any one of those properties is unsatisfactory, a visible image of high quality can be hardly obtained.
In general, those properties largely depend on the characteristics of the toner surface or vicinity thereof, and a variety of improvements of the toner surface have been proposed to enhance those properties. For example, a resin having stable friction-electrostatic chargeability and high hardness is preferably arranged on the surface of toner particles to enhance the powder flowability and durability, while the toner is desired as a whole to have a shock-absorbing property such as flexibility at a satisfactory level.
In the case of an encapsulated toner, a method of forming a double-layered shell around a core material has been developed to improve the surface condition of the encapsulated toner. In this method of forming a double-layered shell, however, the resulting encapsulated toner cannot be effectively provided with the friction-electrostatic chargeability, although the toner can be improved in the flowability, etc. For this reason, the surface of the encapsulated toner is generally deposited with an electrostatic charge modifier such as silica powder to improve the friction-electrostatic chargeability.
Otherwise, a method of coating a condensation polymer such as a styrene polymer on the surface of a toner through condensation polymerization in the presence of benzoyl peroxide has been proposed, as described in Japanese Patent Provisional Publication No. 57(1982)-168254. However, it has been confirmed by the present inventor that the friction-electrostatic chargeability cannot be sufficiently given to the toner even in the case of utilizing the method.